(1) Field of the Invention
The present invention relates to a communication system containing a master station and a plurality of slave stations wherein data sent from the plurality of slave stations is controlled by polling from the master station.
The present invention is applicable, for example, to a B-ISDN terminal system which is connected with a Broadband Integrated Service Digital Network (B-ISDN) network, and contains a plurality of (communication) terminal apparatuses, e.g., telephone, facsimile, video conference terminal and other data terminals.
For future communication network systems using the above B-ISDN, the ATM (Asynchronous Transfer Mode) communication system is proposed wherein data is conveyed on the transmission line by packets each having a predetermined length and called a "cell". Each B-ISDN terminal system detects a vacant cell on the transmission line, and inserts data which is to be transmitted on the cell.
In a B ISDN terminal system connected with such an ISDN network using the ATM system and containing a plurality of communication apparatuses, each of the plurality of communication apparatuses may individually request to send data using one or more cells on the network, and these requests may compete with each other.
FIG. 1 shows an example of arrangement of a B-ISDN terminal system which is connected with a B-ISDN network and comprises a plurality of terminal apparatuses. In FIG. 1, reference numeral 10 denotes a network termination unit, 20.sub.1, 20.sub.2 . . . 20.sub.n-1, 20.sub.n each denote a terminal apparatus, 30 denotes an upstream transmission line, and 40 denotes a downstream transmission line. The network termination unit 10 is provided between the ISDN network and the plurality of terminal apparatuses 20.sub.1, 20.sub.2, . . . 20.sub.n-1, 20.sub.n, and operates as an interface between each terminal apparatus and the ISDN network.
To control timings of the above data transmissions from the plurality of terminal apparatuses, a B-ISDN terminal system is proposed wherein a network termination unit in the system controls data sending operations of all the above plurality of terminal apparatuses by giving allowances to respective terminal apparatuses by polling. Namely, the network termination unit in the B-ISDN terminal system plays a role of a master station, and each of the plurality of terminal apparatuses plays a role of a slave station, regarding the above control of data transmission by polling.
(2) Description of the Related Art
FIG. 2 shows an arrangement of a communication system containing a master station and a plurality of slave stations.
The master station 1 and a plurality of slave stations 2.sub.1, 2.sub.2, . . . 2.sub.n-1, 2.sub.n are connected by transmission lines 3 and 4 each for transmitting signals in directions from each slave station to the master station (upstream line), and from the master station to each slave station (downstream line), respectively.
In the above arrangement, the master station 1 has a polling table (not shown) which points to the addresses of the slave stations in a cyclic order, and each address which is pointed to is renewed after each polling cycle. In the prior art, the master station 1 polls (gives an allowance to send data) equally each slave station in a simple cyclic order using the polling table.
The cycle of the above polling is determined so that a data sending operation from each slave station can be completed without an interference with a signal from an other slave station.
In the conventional communication system as above, it is not considered whether or not each slave station has a request for communication with the master station at the moment of polling.
In the prior art, to solve the above problem, a request assign system is provided wherein the master station 1 sends a signal having a format as shown in FIG. 3 through the downstream line 4 to the plurality of slave stations, and the plurality of slave stations send a signal having a format as shown in FIG. 3 through the upstream line 3 to the master station 1.
In FIG. 3, DL denotes a delimiter which indicates the beginning of a text in each transmission frame, TENO denotes a terminal number to which terminal an allowance to send data is given, and DATA TO SS denotes a data which is to be sent from the master station to one of the plurality of slave stations, DATA FROM SS denotes that data is to be sent from one of the plurality of slave stations toward the master station, and R1, R2, . . . Rn denote request bits from respective slave stations.
The data DATA TO SS includes in itself an address (terminal number) of a slave station to which the data is to be transmitted, and the address in the data DATA TO SS is independent from the above terminal number. The data DATA FROM SS also includes in itself an address to which the data is to be transmitted. Generally, the data DATA TO SS may be generated in the master station, or a cell which has been transmitted in the network, is transmitted as the above data DATA TO SS through the network termination unit 10 in the aforementioned B-ISDN terminal system which is connected with an ISDN network transmitting data by the ATM. Also, generally, the data DATA FROM SS may be processed in the master station, or a packet corresponding to a cell which is to be transmitted in the network, is transmitted as the above data DATA FROM SS in the above B-ISDN terminal system.
In the above request assign system, each 2.sub.i (i=1, 2, . . . n) of the slave stations inserts "1" in the corresponding bit Ri in a frame which is transmitted on the upstream line 3 when the slave station 2.sub.i has a request to send data toward the master station 1.
However, in the above request assign system, each transmission frame on the upstream line includes a region comprised of the above plurality of request bits R1, R2, . . . Rn. The length of the region of the request bits R1, R2, . . . Rn increases with the number of the slave stations, i.e., the transmission efficiency decreases with the number of the slave stations. Further, in the above request assign system, the master station cannot obtain the information on how much data is requested to be transmitted between the master station and each slave station at the moment of polling, and the master station cannot be informed of the priorities of data transmission from respective slave stations.